CaTiO_3－钙钛矿的晶体结构相变、高温流变及其下地幔地球动力学意义

利用高精度的ＭＴＳ材料实验系统，在１４７３－１６７３Ｋ的温度区间和可控热力学环境条件下，对［１００］ｐｃ和［１１０］ｐｃ两个方位的ＣａＴｉＯ＿３－钙钛矿单晶体进行了高温蠕变试验，着重观测了晶体结构相变和高温流变的相关性．研究表明，钙钛矿的晶体结构相变对其高温流变性质有明显影响，斜方晶系钙钛矿蠕变强度大大高于四方和立方晶系钙钛矿，后两者具有很强的高温塑性各向异性．在分析相变过程晶体结构变化特征和观测到的高温流变数据基础上，认为钙钛矿晶体结构相变中发生的位错结构的变化是导致其高温流变性变化的主要物理机制．     

CaTiO3－钙钛矿的晶体结构相变、高温流变及其下地幔地球动力学意义

利用高精度的ＭＴＳ材料实验系统，在１４７３－１６７３Ｋ的温度区间和可控热力学环境条件下，对［１００］ｐｃ和［１１０］ｐｃ两个方位的ＣａＴｉＯ₃－钙钛矿单晶体进行了高温蠕变试验，着重观测了晶体结构相变和高温流变的相关性．研究表明，钙钛矿的晶体结构相变对其高温流变性质有明显影响，斜方晶系钙钛矿蠕变强度大大高于四方和立方晶系钙钛矿，后两者具有很强的高温塑性各向异性．在分析相变过程晶体结构变化特征和观测到的高温流变数据基础上，认为钙钛矿晶体结构相变中发生的位错结构的变化是导致其高温流变性变化的主要物理机制．     

上地幔流变性质的研究进展

本文主要总结了近十多年来橄榄岩流变实验的研究成果,介绍了上地幔流变研究的进展和意义.经过多年的实验研究,橄榄岩流变性质在理论和应用方面都已取得重要突破.人们利用高温高压实验深入研究了熔/流体、铁含量、粒径尺度、温压条件等因素对橄榄石流变性质的影响,得到了表征橄榄石流变特性的幂率本构方程的各参数,并从理论方面探讨了橄榄石等矿物在不同深度处可能存在的变形机制.在应用方面,高温高压流变研究为许多地质现象(如软流圈的成因)的解释和地球动力学(如地幔对流)的模拟提供了实验依据和基础数据,如辉石的粒度效应可以解释上地幔板块边界变形及剪切局域化问题等.尽管对上地幔矿物的流变特性进行了深入地研究,但还有许多问题需要人们进一步厘定,如碳酸盐熔体对橄榄石流变性质的影响,橄榄石变形机制的转化等.     

地壳主要岩石流变参数及华北地壳流变性质研究

岩石流变参数和变形机制是根据断层摩擦和岩石幂次流动本构关系建立岩石圈强度剖面的基础。近 30年来 ,高温高压实验取得了很大进展 ,获得了大量地壳矿物和岩石流变资料。本文系统总结了这些流变实验资料 ,并应用流变数据结合地震震源深度分布 ,对华北地壳流变性质进行了研究。结果表明 ,以花岗岩和低级变质岩为代表的上地壳为脆性破裂 ,其强度受断层摩擦约束 ,以长英质片麻岩为主的中地壳和以中性麻粒岩为主的下地壳上层处于塑性流变状态 ,由干的基性麻粒岩组成的下地壳下层处于脆性向塑性流变的过渡状态。华北地壳的这种物质组成和流变为地壳不同层次的解耦和强震孕育提供了力学条件 ,也构成了不同尺度块体的底边界     

石榴子石的高温塑性：地幔转换带的流变特性

介绍了近年来国际地球物理学界在石榴子石相矿物高温塑性实验研究领域取得的进展和它的意义。这些实验研究进展表明地幔转换带内的富石榴子石岩层的流变强度要大大高于相邻深度的其它矿物相，从而导致地幔转换带流变非均匀性。这种由于矿物组分差异产生的地幔流变性非均匀性对地幔转换带地球动力学特征有着十分重大的影响。首先，石榴子石相矿物在很大程度上决定了那里的强度结构，并控制了地幔转换带的流变性。第二，中、深源地震的成因在很大程度上与石榴子石相矿物流变特性有关，第三，地幔转换带流变的非均匀性将影响到俯冲板块的下插深度和消减过程，最后，对这一课题的未来研究方向和问题作了简单的介绍。     

铁方镁石中金属铁生成的合金反应实验及其地球物理涵义

研究下地幔物质组成和性质是地球动力学的重要课题．铁方镁石(Mg_0.8Fe_0.2O)是下地幔中的主要组成矿物之一,在适当的温度、压强和氧逸度条件下铁方镁石中的二价铁可以通过化学还原反应得到电子形成0价的金属铁．如果有金属生成,则它对下地幔的环境和矿物的物理化学性质具有重要的影响,金属铁会更容易捕获水而降低下地幔中名义无水矿物熔点导致出现部分熔融,金属铁的出现还会影响C、N、H等元素的分布．而实际下地幔中具有等诸多金属元素,会与形成的金属铁形成合金．目前没有关于金属铁形成的合金反应的公开研究,因此本文进行高温高压实验研究铁方镁石中的金属铁生成以及伴随的合金反应,实验温度范围为1573～1773 K,压强范围0～23 GPa．并对高温高压实验后回收的样品进行扫描电镜分析和电子探针分析,通过分析发现采用的钛、铬和碳化钛控制氧逸度的材料会在高温高压实验过程中与铁方镁石形成的金属铁发生合金反应,实验中所有生成的金属铁均以合金的形式存在且合金中的铁含量分布不均匀,同时在样品区域和氧逸度控制材料中出现了反应区,反应区主要成分为氧化镁和控制氧逸度的金属材料氧化...     

中国大陆震源机制深度变化反映的地壳-地幔流变特征

大陆岩石层流变性质是一个重要而又没有完善解决的问题：既存在支持Moho面附近为强地幔-流动弱地壳模式的实验和观测事实，也存在支持流动弱地幔-强地壳模式的观测事实. 本文利用哈佛震源机制矩张量解及有关震源深度资料，对我国震源机制随深度的变化进行分析，发现虽然表浅地震震源机制主应力轴倾角接近水平或竖直，但中、下地壳和上地幔震源机制主应力轴倾角则多样化. 这与某些存在柔性下地壳软弱层地区具有孕震层下缘震源机制主应力轴也接近水平或竖直的特征明显不同. 它可能表明上述两种壳-幔流变模式在我国均存在，对我国岩石层流变性质还需要分区进行更细致的多方面研究.     

岩石圈流变实验研究的进展

岩石高温高压变形实验是了解地球内部物质流动特性的最直接的手段 .本文概述了近年来岩石流变实验研究的一些成果 ,重点总结了水和部分熔融对上地幔顶部橄榄石流变行为的影响 ,以及地壳多相岩石流变实验研究的一些成果 .     

基性岩流变实验揭示出大陆下地壳流变的复杂性

基性麻粒岩流变实验研究对认识大陆下地壳流变、板内构造变形和强震孕育等具有重要意义.大量基性岩流变实验数据表明,基性麻粒岩流变实验研究集中在单相矿物和两相矿物集合体,对天然基性麻粒岩流变研究很有限.基性麻粒岩流变实验结果的影响因素众多,除了实验条件(如温度、压力、应变速率)外,实验样品的矿物组分、样品颗粒粒度、微量水、熔体、矿物反应等都会影响其流变特性,这导致实验结果的复杂性.因此,根据单相或两相矿物集合体流变讨论大陆下地壳流变结构时,存在很多不确定性,不能满足下地壳流变研究的实际需求.而根据端元组分流变参数和经验方程估计由多相矿物组成的麻粒岩的流变强度,只是一种简单近似,无法完全取代麻粒岩流变实验研究.因此,开展多相矿物基性麻粒岩的流变实验,以获得更接近真实大陆下地壳流变的实验数据,并且用于定量研究下地壳流变,是大陆下地壳流变实验未来发展的方向.在基性麻粒岩流变实验研究中存在的科学问题与技术难题是多相矿物流变、高温部分熔融的影响、矿物反应-流变相互作用,这些问题都有待通过高温高压流变实验进行深入研究.     

我国实验岩石力学与构造物理学研究的若干新进展

简述了近年来我国学者在与地震学和地球内部物理学相关的实验岩石力学和构造物理学研究方面的进展．在此领域的进展主要体现在:大量实验以及数学模拟结果丰富了对岩石脆性破裂过程，特别是结构和介质非均匀条件下破裂过程的认识；在非均匀断层的摩擦行为与失稳成核方面取得了一些新结果，揭示了断层滑动行为的复杂性；在岩石脆塑性转换和塑性流变方面取得了一些新结果，特别是在下地壳和上地幔物质的流变性质方面取得了重要进展；在高温高压岩石物理方面取得了一批实验结果, 并应用于地球内部物质的组成和状态的研究．这些结果为深入理解地球内部物质的物理性质、变形机制及地震物理过程提供了有价值的参考资料.      

Laboratory Electrical Conductivity Measurement of Mantle Minerals

Electrical conductivity structures of the Earth’s mantle estimated from the magnetotelluric and geomagnetic deep sounding methods generally show increase of conductivity from 10⁻⁴–10⁻² to 10⁰ S/m with increasing depth to the top of the lower mantle. Although conductivity does not vary significantly in the lower mantle, the possible existence of a highly conductive layer has been proposed at the base of the lower mantle from geophysical modeling. The electrical properties of mantle rocks are controlled by thermodynamic parameters such as pressure, temperature and chemistry of the main constituent minerals. Laboratory electrical conductivity measurements of mantle minerals have been conducted under high pressure and high temperature conditions using solid medium high-pressure apparatus. To distinguish several charge transport mechanisms in mantle minerals, it is necessary to measure the electrical conductivity in a wider temperature range. Although the correspondence of data has not been yet established between each laboratory, an outline tendency of electrical conductivity of the mantle minerals is almost the same. Most of mineral phases forming the Earth’s mantle exhibit semiconductive behavior. Dominant conduction mechanism is small polaron conduction (electron hole hopping between ferrous and ferric iron), if these minerals contain iron. The phase transition olivine to high-pressure phases enhances the conductivity due to structural changes. As a result, electrical conductivity increases in order of olivine, wadsleyite and ringwoodite along the adiabat geotherm. The phase transition to post-spinel at the 660 km discontinuity further can enhance the conductivity. In the lower mantle, the conductivity once might decrease in the middle of the lower mantle due to the iron spin transition and then abruptly increase at the condition of the D″ layer. The impurities in the mantle minerals strongly control the formation, number and mobility of charge carriers. Hydrogen in nominally anhydrous minerals such as olivine and high-pressure polymorphs can enhance the conductivity by the proton conduction. However, proton conduction has lower activation enthalpy compared with small polaron conduction, a contribution of proton conduction becomes smaller at high temperatures, corresponding to the mantle condition. Rather high iron content in mantle minerals largely enhances the conductivity of the mantle. This review focuses on a compilation of fairly new advances in experimental laboratory work together with their explanation.     

Electrical conductivities of pyrolitic mantle and MORB materials up to the lowermost mantle conditions

The electrical conductivities of natural pyrolitic mantle and MORB materials were measured at high pressure and temperature covering the entire lower mantle conditions up to 133 GPa and 2650 K. In contrast to the previous laboratory-based models, our data demonstrate that the conductivity of pyrolite does not increase monotonically but varies dramatically with depth in the lower mantle; it drops due to high-spin to low-spin transition of iron in both perovskite and ferropericlase in the mid-lower mantle and increases sharply across the perovskite to post-perovskite phase transition at the D″ layer. We also found that the MORB exhibits much higher conductivity than pyrolite. The depth–conductivity profile measured for pyrolite does not match the geomagnetic field data below about 1500-km depth, possibly suggesting the existence of large quantities of subducted MORB crust in the deep lower mantle. The observations of geomagnetic jerks suggest that the electrical conductivity may be laterally heterogeneous in the lowermost mantle with high anomaly underneath Africa and the Pacific, the same regions as large low shear-wave velocity provinces. Such conductivity and shear-wave speed anomalies are also possibly caused by the deep subduction and accumulation of dense MORB crust above the core–mantle boundary.     

Mantle dynamics: The strong control of the spinel-perovskite transition at a depth of 660 km

The strong control that the endothermic phase change from spinel to perovskite and magnesiowüstite at a depth of 660 km has on mantle convection is discussed. The phase transition determines the morphology and length scales of upflow and downflow structures and, through retardation of sinking slabs, can cause an avalanche phenomenon involving rapid flushing of cold upper mantle material down to the base of the lower mantle. The phase change significantly heats plumes that rise from the lower mantle and penetrate into the upper mantle. The exothermic phase change from olivine to spinel at a depth of 400 km in the mantle mitigates the effects of the dynamically and thermally dominant endothermic phase transition.     

How flat is the lower-mantle temperature gradient?

The temperature gradient in the lower mantle is fundamental in prescribing many transport properties, such as the viscosity, thermal conductivity and electrical conductivity. The adiabatic temperature gradient is commonly employed for estimating these transport properties in the lower mantle. We have carried out a series of high-resolution 3-D anelastic compressible convections in a spherical shell with the PREM seismic model as the background density and bulk modulus and the thermal expansivity decreasing with depth. Our purpose was to assess how close under realistic conditions the horizontally averaged thermal gradient would lie to the adiabatic gradient derived from the convection model. These models all have an endothermic phase change at 660 km depth with a Clapeyron slope of around −3 MPa K⁻¹, uniform internal heating and a viscosity increase of 30 across the phase transition. The global Rayleigh number for basal heating is around 2×10⁶, while an internal heating Rayleigh number as high as 10⁸ has been employed. The pattern of convection is generally partially layered with a jump of the geotherm across the phase change of at most 300 K. In all thermally equilibrated situations the geothermal gradients in the lower mantle are small, around 0.1 K km⁻¹, and are subadiabatic. Such a low gradient would produce a high peak in the lower-mantle viscosity, if the temperature is substituted into a recently proposed rheological law in the lower mantle. Although the endothermic phase transition may only cause partial layering in the present-day mantle, its presence can exert a profound influence on the state of adiabaticity over the entire mantle.     

Experimental Studies of Shear Deformation of Mantle Materials: Towards Structural Geology of the Mantle

—A brief outline is given on experimental studies carried out in the Minnesota Mineral and Rock Physics Laboratory of microstructural evolution and rheology of mantle mineral aggregates or their analogues, using a simple shear deformation geometry. A simple shear deformation geometry allows us to unambiguously identify controlling factors of microstructural evolution and to obtain large strains at high pressures and temperatures, and thus provides a unique opportunity to investigate the "structural geology of the mantle." We have developed a simple shear deformation technique for use at high pressures and temperatures (pressure up to 16 GPa and temperature up to 2000 K) in both gas-medium and solid-medium apparati. This technique has been applied to the following mineral systems (i) olivine aggregates, (ii) olivine basaltic melt, (iii) CaTiO₃ perovskite aggregates. The results have provided important data with which to understand the dynamics of the earth’s mantle, including the geometry of mantle convection, mechanisms of melt distribution and migration beneath mid-ocean ridges, and the mechanisms of shear localization. Limitations of laboratory studies and future directions are also discussed.     

Silicate perovskite analogue ScAlO3: temperature dependence of elastic moduli

The elastic moduli of ScAlO₃ perovskite, a very close structural analogue for MgSiO₃ perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al₂O₃ measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO₃ perovskite, the value of (∂K_S/∂T)_P is −0.033 GPa K⁻¹ and (∂G/∂T)_P is −0.015 GPa K⁻¹. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂K_S/∂T)_P| is typically about twice |(∂G/∂T)_P|. The value of (∂K_S/∂T)_P for ScAlO₃ is intermediate between those inferred less directly from V(P,T) studies of Fe-free and Fe- and Al-bearing MgSiO₃ perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P–V–T equation of state of (Mg,Fe)SiO₃ perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO₃ perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂G/∂T)|_P for ScAlO₃ is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO₃ perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO₃ perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679].

Combining the results from the previous studies and current measurements on ScAlO₃ perovskite, we extracted the parameters (q and γ₀) needed to fully specify its Mie–Grüneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of K_S(T), unlike V(P,T) data, tightly constrain the value of q. It is concluded that ScAlO₃ has ‘normal’ γ₀ (1.3) and high q (3.6). The high value of q indicates that ScAlO₃ has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).     

The energetics of aluminum solubility into MgSiO3 perovskite at lower mantle conditions

Experiments [T. Irifune (1994) Nature 370, 131–133; E. Ito et al. (1998) Geophys. Res. Lett. 25, 821–824; A. Kubo, M. Akaogi (2000) Phys. Earth Planet. Int. 121, 85–102] indicate that (Mg,Fe)SiO₃ perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al₂O₃ in the Earth’s lower mantle. Aiming to better understand the effects of Al₂O₃ on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO₃(perovskite)–Al₂O₃(corundum)–MgO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P=25–100 GPa, and T=1000–2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO₃ perovskite in terms of reactions involving MgSiO₃, MgO, and Al₂O₃, in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620–622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617–625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al₂O₃ and MgO components in solid solution with MgSiO₃, as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancy-forming substitution accounts for 3–4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO₃ perovskite.     

On the 650-km seismic discontinuity

The pressure-temperature conditions and the variations of both density and bulk sound velocity in the vicinity of the 650-km discontinuity have been compared with those calculated for the phase transitions in both the olivine and the pyroxene-garnet components of the mantle material. These studies suggest that the mantle below about 650 km is composed primarily of perovskite phase, as distinct from the olivine-rich upper mantle. Thus, the “650-km” discontinuity is not likely to be associated with any of the equilibrium phase boundaries observed in olivine, pyroxene, and garnet, and is proposed instead to be a chemical change. It is suggested that the following factors may be responsible for chemical separation: the pyroxene-garnet component transforms to much denser phases possessing the ilmenite and perovskite structures before the breakdown of the spinel phase into a mixture of perovskite plus rocksalt phases. The perovskite phase is also much denser than the rocksalt phase and the two phases may not form a gravitationally stable mixture. Thus, the denser phases may tend to sink to or stay at the deep part of the mantle, causing chemical separation. Possible separation processes are discussed and the supporting observations are presented.     

上地幔橄榄岩流变性研究新进展及其地球动力学意义

本文系深部地幔流性研究进展综述文章之一，重点介绍了近年来国际地球物理学界在涉及橄榄岩上地幔流变性的下述三方面取得的进展：（１）化学环境因素对橄榄石高温塑性的影响；（２）高温高压下水在橄榄石晶体中的溶解度和赋存状态；（３）橄榄石多晶体的变形机制转变，并对由此得出的地球动力学意义作了讨论。